1) Field of the Invention
The present invention relates to a tire air pressure monitoring system for monitoring an air pressure of each of tires of a vehicle, and more particularly to the reduction of electric power dissipation (consumption) in the tire air pressure monitoring system.
2) Description of the Related Art
So far, there have been proposed various types of systems designed to monitor an air pressure of each of tires mounted on a vehicle. One approach is a tire air pressure monitoring system in which a sensor unit composed of a sensor for detecting a tire air pressure and a transmitter for transmitting the detection value thereof is attached onto each of tires of a vehicle while a monitoring unit is mounted in the vehicle body side so that the sensor unit transmits a detection signal indicative of an air pressure detection value on each tire unit and the monitoring unit issues a warning in a case in which the air pressure detection value from the sensor unit falls below a predetermined value.
For example, Japanese Patent Laid-Open No. 2001-105811 (corresponding to U.S. Pat. No. 6,430,484) discloses a tire air pressure monitoring system in which a monitoring unit mounted in a vehicle body side is placed into a waking-up mode in order to lower the electric power dissipation at the tire air pressure detection when a vehicle engine falls into a stop condition. This waking-up mode signifies that the monitoring unit is alternately set in a receivable condition and an unreceivable condition with respect to a signal transmitted from a sensor unit. In a case in which the monitoring unit is in the unreceivable condition, the system stops the power supply to principal equipment, such as a receiver, constituting the monitoring unit, thereby achieving the reduction of the electric power dissipation in the monitoring unit.
However, the foregoing conventional tire air pressure monitoring system is designed such that the monitoring unit is set in the transmitted signal receivable condition at a constant interval and, if a transmitted signal from the sensor unit is received in the receivable condition, the transmitted signal is picked up, while if the transmitted signal from the sensor unit is absent in the receivable condition which continues for a constant period of time, the receivable condition is switched into the unreceivable condition (sleep mode). That is, in the foregoing conventional tire air pressure monitoring system, the monitoring unit is made to be set in the receivable condition at a constant interval irrespective of the presence or absence of the transmitted signal. This leads to the useless consumption of electric power and is remote from a sufficient reduction of electric power dissipation.